Several alphatic ketones to which humans are exposed are known to potentiate the toxicities produced by various xenobiotics including CHC13 and CC14. We have purified to apparent homogeneity the major form of cytochrome P-450 induced by methyl-butyl ketone (MBK) and demonstrated that it is similar to the major form induced by phenobarbital (PB). They have the same apparent molecular weights, are digested similarly by chymotrypsin and V-8 protease, and both catalyze the metabolism of CHC13 to COC12. Initial studies with acetone demonstrate that it also increases cytochromes P-450 levels and results in an increased rate of microsomal metabolism of CHC13 to COC12. therefore, MBK and acetone appear to potentiate the hepatotoxicity of CHC13 and possibly other compounds by increasing the rate of their metabolism to toxic metabolites.